Improved fracture toughness of carbon fibre/epoxy composite laminates using dissolvable thermoplastic fibres

This paper reports on a novel toughening concept based on dissolvable phenoxy fibres, which are added at the interlaminar region in a carbon fibre/epoxy composite. The composites were prepared by resin infusion of carbon fibre fabric with the phenoxy introduced as a chopped fibre interleaf between the carbon fibre plies. The thermoplastic phenoxy fibre dissolved in the epoxy during curing at elevated temperatures and a phase separated morphology with phenoxy-rich secondary phase was formed upon curing. It was found that the average Mode-I fracture toughness value, G_1c increased tenfold with only 10 wt.% (with regard to the total matrix content) phenoxy fibre added. Other properties such as Young’s modulus, tensile strength and thermal stability were not adversely affected. The mechanical and thermal properties of the neat epoxy–phenoxy blends were also studied for comparison.     

Tensile and fatigue characterisation of textile cotton waste/polypropylene laminates

Short fibre based cotton flocks from end-of-life jeans fabric (denim twill weave) were introduced in an amount of 16 wt.% in a polypropylene (PP) matrix using a specifically designed manufacturing process to preserve as much as possible the properties of the cotton waste during injection moulding. This involved a first phase of binding the cotton flocks on polyvinyl acetate (PVAc) support, then pelletizing them with PP and finally extruding the final composite. The resulting composites were subjected to morphological, tensile and fatigue characterisation with stress levels from 50 to 90% of ultimate tensile strength. Results indicated that injection moulding offered a sufficient uniformity of properties to the composite, albeit with some occurrence of pull-out during loading. In particular, the tensile performance exceeded that of the pure matrix in a measure compatible with the amount of fibres introduced. In addition, tensile fatigue loading up to 5000 cycles evidenced a limited amount of degradation for maximum applied stresses up to 70% of composite tensile strength.     

Composites based on polypropylene and short wool fibres

Isotropic polypropylene-based composites, containing up to 60 wt.% of well dispersed wool fibres, were successfully prepared by melt blending in an internal batch mixer. The addition of a maleinized polypropylene compatibilizer was investigated in order to improve fibre/matrix adhesion. Morphology, thermal and mechanical properties of the ensuing composites were investigated focusing the attention on fibre length and their distribution as well as on fibre/matrix interaction. Data from mechanical analysis were compared with theoretical models and with the mechanical characteristics of a composite made of polypropylene with uncut aligned wool fibres.     

Characterisation of cotton fibre-reinforced geopolymer composites

This paper describes the physical, mechanical and fracture behaviour of fly-ash based geopolymer reinforced with cotton fibres (0.3–1.0 wt%). Results show that the appropriate addition of cotton fibres can improve the mechanical properties of geopolymer composites. In particular, the flexural strength and the fracture toughness increase at an optimum fibre content of 0.5 wt%. However, as the fibre content increases, the density of geopolymer composites decreases due to an increase in porosity and tendency of fibre agglomeration.     

Composite materials based on wood and nylon fibre

Nylon-wood fibre and polypropylene-wood fibre composite materials were manufactured without any additives to determine the effects of wood fibre on the mechanical properties of the different composites. The raw materials used were eucalypt hardwood fibre, Nylon fibre obtained from stockings, and polypropylene (PP) pellets. A hot press technique was used to manufacture the composite materials, and improvements in the manufacturing methods are suggested. Tests were carried out on the manufactured boards to determine tensile strength and modulus of elasticity. Fracture surfaces were examined using scanning electron microscopy to investigate failure mechanisms. An increase in tensile strength and modulus of elasticity was observed in wood fibre/Nylon matrix composites, indicating that interfacial bonding occurred between these two phases. Bundles of wood fibres with internal voids prevent achieving maximum mechanical properties. The tensile strength of the PP based composites decreased significantly with increasing wood fibre content.     

Procedural influences on compression and injection moulded cellulose fibre-reinforced polylactide (PLA) composites: Influence of fibre loading,fibre length,fibre orientation and voids

The influence of fibre loading (20, 30, 40 mass%), fibre fineness, and the processing procedure (compression moulding – CM and injection moulding – IM) on the tensile and impact strength of lyocell/PLA composites was examined. The results revealed a significantly higher tensile and impact strength for CM composites compared to IM composites. An increase in strength up to a fibre loading of 40% was determined for CM composites, while for IM composites the highest values were measured at a fibre loading of 30%. Composites were investigated for their void content, fibre orientation, fibre length and process-induced fibre damage. A better fibre/matrix adhesion and compaction of IM composites was found while fibre orientation as well as mechanical properties of extracted fibres show no significant differences between CM and IM composites. The different mechanical characteristics of CM and IM samples are attributed predominantly to the fibre aspect ratio and the distribution of voids.     

Preparation, structure and mechanical properties of all-hemp cellulose biocomposites

All-hemp (Cannabis Sativa L.) cellulose composites were prepared by a mechanical blending technique followed by hot pressing and water–ethanol regeneration. The alkali treated fibres were ground and sieved to a size ranging from 45 μm to 500 μm. Introduction of fibres into 12% w/v cellulose N-methyl-morpholine-N-oxide (NMMO) solution was performed with low solution viscosity at 100 °C. The solid mixtures were cut and heat pressed between heated glass and PTFE plates at 85 °C to obtain a flat smooth-surfaced composite sheet of approximately 0.2 mm thickness. The cellulose was regenerated in a 50:50 water–ethanol mixture that subsequently removed NMMO and stabilizer (Irganox 1010, Ciba) from the composite. FTIR and X-ray diffraction measurements were performed to investigate the structural change of cellulose from fibre into partially regenerated composite. Composition and thermal stability of composites were investigated using thermogravimetry. A broadening of the scattering of the main crystalline plane (0 0 2) and a depression of the maximum degradation temperature of fibre were observed. The observations revealed a structural change in the fibres. The mechanical properties of composites depended on size, surface area, crystallinity and the structural swelling of fibres.     

Jute/polypropylene composites I. Effect of matrix modification

Natural fibre-reinforced polymers can exhibit very different mechanical performances and environmental aging resistances depending on their interphase properties, but most studies have been focused on fibre surface treatment. Here, investigations of the effect of maleic anhydride grafted polypropylene (MAHgPP) coupling agents on the properties of jute fibre/polypropylene (PP) composites have been considered with two kinds of matrices (PP1 and PP2). Both mechanical behaviour of random short fibre composites and micro-mechanical properties of single fibre model composites were examined. Taking into account interfacial properties, a modified rule of mixture (ROM) theory is formulated which fits well to the experimental results. The addition of 2 wt% MAHgPP to polypropylene matrices can significantly improve the adhesion strength with jute fibres and in turn the mechanical properties of composites. We found that the intrinsic tensile properties of jute fibre are proportional to the fibre’s cross-sectional area, which is associated with its perfect circle shape, suggesting the jute fibre’s special statistical tensile properties. We also characterised the hydrophilic character of natural fibres and, moreover, humidity environmental aging effects. The theoretical results are found to coincide fairly well with the experimental data and the major reason of composite tensile strength increase in humidity aging conditions can be attributed to both improved polymer–matrix and interfacial adhesion strength.     

Dynamic mechanical and thermal properties of MAPE treated jute/HDPE composites

The present paper summarizes an experimental study on the mechanical and viscoelastic behavior of jute fibre reinforced high density polyethylene (HDPE) composites. Variations in mechanical strength, storage modulus (E′), loss modulus (E″) and damping parameter (tan δ) with the addition of fibres and coupling agents were investigated. It was observed that the tensile, flexural and impact strengths increased with the increase in fibre loading upto 30%, above which there was a significant deterioration in the mechanical strength. Further, the composites treated with MAPE showed improved properties in comparison to the untreated composites. Dynamic mechanical analysis data showed an increase in the storage modulus of the treated composites The tan δ spectra presented a strong influence of fibre content and coupling agent on the α and γ relaxation process of HDPE. The thermal behavior of the composites was evaluated from TGA/DTG thermograms. The fibre–matrix morphology in the treated composites was confirmed by SEM analysis of the tensile fractured specimens. FTIR spectra of the treated and untreated jute fibres was also studied to ascertain the existence of type of interfacial bonds.     

Shear strength and fracture toughness of carbon fibre/epoxy interface: effect of surface treatment

Textile-reinforced composites have become increasingly attractive as protection materials for various applications, including sports. In such applications it is crucial to maintain both strong adhesion at fibre–matrix interface and high interfacial fracture toughness, which influence mechanical performance of composites as well as their energy-absorption capacity. Surface treatment of reinforcing fibres has been widely used to achieve satisfactory fibre–matrix adhesion. However, most studies till date focused on the overall composite performance rather than on the interface properties of a single fibre/epoxy system. In this study, carbon fibres were treated by mixed acids for different durations, and resulting adhesion strength at the interface between them and epoxy resin as well as their tensile strength were measured in a microbond and microtensile tests, respectively. The interfacial fracture toughness was also analysed. The results show that after an optimum 15–30 min surface treatment, both interfacial shear strength and fracture toughness of the interface were improved alongside with an increased tensile strength of single fibre. However, a prolonged surface treatment resulted in a reduction of both fibre tensile strength and fracture toughness of the interface due to induced surface damage.     

Application of lignin as natural adhesion promoter in cotton fibre-reinforced poly(lactic acid) (PLA) composites

This study investigated how lignin—used as a natural adhesion promoter in biodegradable, thermoplastic cotton fibre-reinforced composites—influences the composites’ mechanical properties. Composites with fibre mass proportions of 40% were produced by compression moulding. Poly(lactic acid) (PLA), a biopolymer, served as matrix. Cotton/PLA composites with and without lignin content were manufactured. As reference samples of bast fibre-reinforced composites, kenaf/PLA composites were produced under the same conditions. The composites were tested for stiffness, tensile strength, elongation at break and impact strength. Fractured surfaces were analysed using scanning electron microscopy (SEM). The results of the composite investigations showed that the addition of lignin has an influence on the cotton/PLA composite characteristics. SEM investigations showed that the adhesion between fibre and matrix could be improved by the addition of lignin. Tensile characteristics like tensile strength and Young’s modulus could be improved clearly, while the impact properties were decreased.     

Thermal and mechanical properties of cotton fabric-reinforced geopolymer composites

As natural fibres, cotton fabrics (CF) offer good opportunities as reinforcement material for geopolymer composites as they have good intrinsic mechanical properties. This article presents thermal and mechanical properties of CF-reinforced geopolymer composites containing up to 4.1 wt% CF. Thermo-gravimetric analysis was conducted to characterise their thermal performance and their mechanical properties, such as flexural strength, fracture toughness, flexural modulus and impact strength were evaluated. Results show that the enhancement of mechanical properties was achieved at an optimum fibre content of 2.1 wt%. Results of thermal analysis show that fly-ash based geopolymer can prevent the degradation of cotton fabric at elevated temperatures.     

The influence of fibre microstructure on fibre breakage and mechanical properties of natural fibre reinforced polypropylene

The aim of the study was to investigate the influence of fibre morphology of different natural fibres on the composites mechanical properties and on the fibre breakage due to extrusion process. The composite materials were manufactured using LTF (long fibre thermoplastic) extrusion and compression moulding and the used fibres were sisal, banana, jute and flax, and the matrix was a polypropylene. The results showed that sisal composites had the best impact properties and the longest fibres after the extrusion. Generally, the composites flexural stiffness was increased with increased fibre content for all fibres, being highest for flax composites. The flexural strength was not affected by the addition of fibres because of the low compatibility. The addition of 2 wt.% maleated polypropylene significantly improved the composites properties. Unlike the other three fibres, flax fibres were separated into individual elementary fibres during the process due to enzymatic retting and low lignin content.     

Processing and properties of poly(ε-caprolactone)/carbon nanofibre composite mats and films obtained by electrospinning and solvent casting

Composite materials based on poly(ε-caprolactone) (PCL) and carbon nanofibres (CNFs) were processed by solvent casting and electrospinning. The main objective was to investigate the effects of the CNFs on the microstructural, thermal and mechanical properties of the PCL matrix composites processed by two different routes. The hybrid materials obtained with different CNF content (1, 3 and 7 wt%) were analysed by electron microscopy (FESEM), differential scanning calorimeter (DSC), thermogravimetry (TGA) and mechanical testing. The composite films showed a good dispersion in the PCL matrix while electrospun samples were consisted of homogeneous and uniform fibres up to 3 wt% CNFs with average fibre diameter ranged between 0.5 and 1 μm. Composite films and mats revealed an increased crystallization temperature with respect to the neat PCL matrix. Mechanical properties of solvent cast films and electrospun mats were assessed by uniaxial tensile tests. A stiffness increase was achieved in PCL films depending on the CNF content, while mechanical properties of mats were only slightly affected by CNF introduction.     

Effect of pectin and hemicellulose removal from hemp fibres on the mechanical properties of unidirectional hemp/epoxy composites

The objective of this study was to investigate the effect of pectin and hemicellulose removal from hemp fibres on the mechanical properties of hemp fibre/epoxy composites. Pectin removal by EDTA and endo-polygalacturonase (EPG) removed epidermal and parenchyma cells from hemp fibres and improved fibre separation. Hemicellulose removal by NaOH further improved fibre surface cleanliness. Removal of epidermal and parenchyma cells combined with improved fibre separation decreased composite porosity factor. As a result, pectin removal increased composite stiffness and ultimate tensile strength (UTS). Hemicellulose removal increased composite stiffness, but decreased composite UTS due to removal of xyloglucans. In comparison of all fibre treatments, composites with 0.5% EDTA + 0.2% EPG treated fibres had the highest tensile strength of 327 MPa at fibre volume content of 50%. Composites with 0.5% EDTA + 0.2% EPG → 10% NaOH treated fibres had the highest stiffness of 43 GPa and the lowest porosity factor of 0.04.     

Novel automated method for evaluating the morphological changes of cellulose fibres during extrusion-compounding of plastic–matrix composites

A novel method based on fluorescence optical microscopy has been developed for determining the fibre geometrical changes occurring during the melt processing of cellulose-reinforced composites, which are known to be closely related with composite properties. Determination of these changes is still a tedious and challenging task because existing methods are not well developed yet. The novel method proved its ability for explaining the screw configuration effects on the attrition bore by the fibres during extrusion-compounding of plastic–matrix composites. The percentage of fibres longer than the critical length parameter was revealed to highlight the mechanical degradation of fibres during compounding. The percentage of fines exhibited the clearest correlation with the differences in fibre content of composites. Relationships found between composite tensile properties and fibre characterization parameters revealed the ability of the novel method for explaining the effects of composition and processing on composite properties.     

Characteristics of Hermès flax fibres as a function of their location in the stem and properties of the derived unidirectional composites

The tensile mechanical properties of flax fibres from the Hermès variety are estimated according to their diameter and their location in the stems. The large scattering of these properties is ascribed to the variation of the fibre size along its longitudinal axis, as revealed by SEM observations. The higher values of the mechanical properties for the fibres issued from the middle of the stems are associated with the chemical composition of their cell walls. The mechanical properties of unidirectional flax fibre/epoxy matrix composites are studied as a function of their fibre content. The properties of the composites are lower than those expected from single fibre characteristics.     

Thermomechanical properties of bio-based composites made from a lactic acid thermoset resin and flax and flax/basalt fibre reinforcements

Low viscosity thermoset bio-based resin was synthesised from lactic acid, allyl alcohol and pentaerythritol. The resin was impregnated into cellulosic fibre reinforcement from flax and basalt and then compression moulded at elevated temperature to produce thermoset composites. The mechanical properties of composites were characterised by flexural, tensile and Charpy impact testing whereas the thermal properties were analysed by dynamic mechanical thermal analysis (DMTA) and thermogravimetric analysis (TGA). The results showed a decrease in mechanical properties with increase in fibre load after 40 wt.% for the neat flax composite due to insufficient fibre wetting and an increase in mechanical properties with increase fibre load up to 60 wt.% for the flax/basalt composite. The results of the ageing test showed that the mechanical properties of the composites deteriorate with ageing; however, the flax/basalt composite had better mechanical properties after ageing than the flax composite before ageing.     

Extraction and preparation of bamboo fibre-reinforced composites

Natural plant fibre composites have been developed for the production of a variety of industrial products, with benefits including biodegradability and environmental protection. Bamboo fibre materials have attracted broad attention as reinforcement polymer composites due to their environmental sustainability, mechanical properties, and recyclability, and they can be compared with glass fibres. This review classifies and describes the various procedures that have been developed to extract fibres from raw bamboo culm. There are three main types of procedures: mechanical, chemical and combined mechanical and chemical extraction. Composite preparation from extracted bamboo fibres and various thermal analysis methods are also classified and analysed. Many parameters affect the mechanical properties and composite characteristics of bamboo fibres and bamboo composites, including fibre extraction methods, fibre length, fibre size, resin application, temperature, moisture content and composite preparation techniques. Mechanical extraction methods are more eco-friendly than chemical methods, and steam explosion and chemical methods significantly affect the microstructure of bamboo fibres. The development of bamboo fibre-reinforced composites and interfacial adhesion fabrication techniques must consider the type of matrix, the microstructure of bamboo and fibre extraction methods.     

Effects of alkali treatment and elevated temperature on the mechanical properties of bamboo fibre–polyester composites

Bamboo fibre reinforced composites are not fully utilised due to the limited understanding on their mechanical characteristics. In this paper, the effects of alkali treatment and elevated temperature on the mechanical properties of bamboo fibre reinforced polyester composites were investigated. Laminates were fabricated using untreated and sodium hydroxide (NaOH) treated (4–8% by weight) randomly oriented bamboo fibres and tested at room and elevated temperature (40, 80 and 120 °C). An improvement in the mechanical properties of the composites was achieved with treatment of the bamboo fibres. An NaOH concentration of 6% was found optimum and resulted in the best mechanical properties. The bending, tensile and compressive strength as well as the stiffness of this composite are 7, 10, 81, and 25%, respectively higher than the untreated composites. When tested up to 80 °C, the flexural and tensile strength are enhanced but the bending stiffness and compressive strength decreased as these latter properties are governed by the behaviour of resin. At 40 and 80 °C, the bond between the untreated fibres and polyester is comparable to that of treated fibres and polyester which resulted in almost same mechanical properties. However, a significant decrease in all mechanical properties was observed for composites tested at 120 °C.